Heavy-duty high-voltage semiconductor converters are becoming increasingly popular in various fields of application in science and technology. They are used in AC electric drives with induction and synchronous motors, in stabilized power supplies, in plasma-jet electron-ion technology, in power supply systems for electrophysical research, and other various fields.
Application of powerful high-voltage semiconductor converters in these fields offers the advantages of improved technological processes, automation of such processes, efficient use of electric power, minimization of losses of this electric power.
One common feature of heavy-duty converters rated for 3-20 kV consists in the use of series-connected semiconductor rectifiers. Such semiconductor rectifiers are extremely sensitive to overloads because of their small size, minor heat capacity, and insufficient reserve of electrical strength. Special techniques and means for protection of semiconductor rectifiers should be developed for efficient troubleshooting as part of their maintenance.
The problem of developing failure detection devices has become a serious challenge and cannot be solved without making use of the repertoire of modern power converters. Such failure detection devices should be capable of preventing emergency processes in heavy-duty high-voltage converters, minimizing the time needed for trouble hunting, providing for preventative maintenance by few unskilled personnel, resulting in lower costs of maintenance.
Known in the art are devices for failure detection in series-connected semiconductor rectifiers (cf., for example, the USSR Inventor's Certificate No. 481 963, Cl. H 02 H 7/10; the USSR Inventor's Certificate No. 620 919, Cl. H 02 H 7/10) which comprise, in order to determine if the rectifier is broken down or not, a measuring element incorporated into the diagonal of an unbalanced bridge composed of a circuit of series-connected monitored rectifiers, each rectifier being shunted by a resistor and a resistor voltage divider. These devices realize the so-called passive failure detection technique for monitoring the series-connected semiconductor rectifiers. This technique is based on detecting a signal produced in the bridge diagonal when the balance of this bridge is upset by the breakdown of a rectifier. The accuracy and reliability of such devices are not satisfactory, since much depends on the operating conditions of the semiconductor rectifier and the magnitude of the supply voltage. Such devices, therefore, have to be individually adjusted, and, since they realize the passive detection technique, they cannot be used for failure detection in semiconductor rectifiers before the supply voltage is applied to these rectifiers. In addition, such testing devices have an extremely complicated maintenance procedure.
Also known in the art is a device (cf., for example, British Pat. No. 1,540,300 Cl. H 02 M 1/18) which also realizes the passive testing method for failure detection in series-connected semiconductor rectifiers by measuring the forward and reverse voltages across these rectifiers. This device uses optoelectronic sensors whose number is equal to that of the series-connected semiconductor rectifiers. In this manner the protection system is conductively isolated from the power portion of the converter.
The disadvantages of this device consist in that it is too complicated, requires additional logical processing of information fed from optoelectronic sensors, and comprises a large number of elements located near the rectifiers, which impairs its reliability. Faulty rectifiers cannot be detected before the supply voltage is applied.
Also known in the art is a device for controlling and failure detection of series-connected rectifiers (cf., for example, the USSR Inventor's Certificate No. 256,049, Cl. 21 d2 12/03 H 2M), comprising a pulse generator having the first lead thereof connected to a terminal of the circuit composed of series-connected supply windings of current transformers, whose number is equal to that of the rectifiers, the input winding of each transformer being connected to a cathode of a respective rectifier, and RC-circuits whose number is equal to that of the rectifiers. The noise immunity of the device is improved by installing additional transformers whose number is equal to that of the rectifiers, the cores of the first and additional transformers being arranged in symmetry to the series-connected supply windings.
This device can, in principle, provide failure detection in rectifiers irrespective of the magnitude of the supply voltage of the converter.
But the device is deficient in that the circuitry and design are overcomplicated. In addition, it is not sufficiently reliable because of the large number of series-connected elements.